This invention relates to an information recording method and an information reproducing method employing a storage type moving picture medium, such as an optical disc or a magnetic tape, a method and apparatus for picture encoding and decoding which may be applied to a teleconferencing system, a moving picture telephone system or an information transmission/reception system for a telecasting equipment, and a picture recording medium.
Representative of the motion compensation prediction coding add decoding methods is a standard system for storage type moving picture coding, known as MPEG1, formulated at ISO-IEC/JTC1/SC2/WG11, commonly termed Motion Picture Experts Group (MPEG).
With the MPEG1, a number of moving pictures a unit are grouped into a group of pictures (GOP). A constructional example of GOP is shown in FIG. 1, in which there are three types of pictures in the GOP, that is an I-picture or intra-coded picture, a P-picture or a predictive-coded picture and a B-picture or a bi-directionally predictive-coded picture. The I-picture is an intra-frame coded picture. The P-picture is an inter-frame coded picture, that is a picture obtained by motion prediction from the recently decoded and reproduced past I-picture or the P-picture and encoding of the prediction error. The B-picture is a bi-directionally prediction-coded picture obtained by motion prediction from both the past reproduced picture and a future picture yet to be reproduced and encoding of the prediction error. The GOP is made up of one or more I-picture(s) and 0 or plural non-I picture(s). In FIG. 1, the I-picture, P-picture and the B-picture are indicated by letters I, P and B, respectively. Besides, in FIG. 1, arrows interconnecting the pictures denote the directions of motion prediction.
In addition, in MPEG1, GOP headers 1, 2, 3, . . . N are appended to GOP 1 , GOP 2, GOP 3, . . . GOP N, as shown in FIG. 2, for enabling random accessing (reproduction from an arbitrary point) on the GOP basis. Consequently, the picture decoding device detects a GOP start code of a designated GOP in a coded input string of data, or an input bitstream, and causes the decoding to be started at the leading I-picture of the GOP to enable the reproduction from an arbitrary point in the sequence of moving pictures.
It is noted that two flags, namely a closed GOP flag and a broken link GOP flag, are appended to the GOP header.
The closed GOP flag is set during encoding for indicating that a P-picture of a past GOP, which is a P-picture indicated by hatching in the case of FIG. 1, is not used for predicting first plural B-pictures, herein B-pictures of two frames. A GOP with the closed GOP flag set is a completely independent GOP.
The broken link flag is set when a bitstream other than a closed GOP is sliced and linked by editing. Thus, when a broken link flag is set for a GOP, the picture decoding device recognizes that several leading B-pictures cannot be displayed for the GOP.
These two flags are relevant to several leading B-pictures of a given GOP. If random accessing is had to a designated GOP, that is if the reproduction is made at an arbitrary point in the sequence of moving pictures, the leading B-picture(s) cannot be displayed in the worst case. However, reproduction may be made as from the I-pictures.
As for the detailed code and the grammar of the bitstream concerning the GOP, reference is had to ISO/IEC 11172-2 which is a standard material of MPEG1.
In U.S. Pat. No. 5,191,436, there is disclosed a technique of setting an edit flag when a bitstream is sliced and sliced portions are linked together by way of editing.
Recently, a system of encoding interlaced pictures, such as pictures of the NTSC or PAL system for television, is under investigation in the MPEG2 formulated in the wake of MPEG1. With the MPEG2, the processing units of the pictures to be encoded are adaptively changed over from the frame-structure unit to the field-structure unit or vice versa to produce a plurality of decoded and reproduced past pictures.
With the MPEG2, similarly to the MPEG1, there are three pictures in a GOP, that is an I-picture, a P-picture and a B-picture, each of which may be of a frame structure or a field structure.
If random accessing is made on the GOP basis with the MPEG2, the following problem is met when the leading I-picture of a GOP is of the field structure,
That is, if a picture p13 now to be encoded is of a field structure, motion prediction may be made from decoded and reproduced past two fields, namely a field p06 and a field i12, as shown in FIG. 3, If the prediction is made from the field p06, the GOP 1 ceases to be independent, such that random accessing cannot be made to the GOP 1. The reason is that, when random accessing is had to the GOP 1, the field p06, which should be referred to during motion compensation required for reproducing the field p13, is not yet reproduced. In FIG. 3, i, p and b denote an I-picture, a P-picture and a B-picture, respectively, while slanted broken lines indicate lines of interruption between neighboring GOPs.